Barium crown glasses are well known in the optical glass art. They are designed to provide a relatively high refractive index (n.sub.D) in conjunction with a high dispersion (Nu) value. In general, a barium crown glass will have a n.sub.D value greater than 1.55 and an Nu value greater than 55.
Barium crown glasses find particular application in molded optical glass elements. These include optical components used in optical measuring instruments and lens systems for light projectors.
It is well recognized that barium borosilicate glasses tend to have chemical durability problems. A considerable amount of composition study has gone into seeking a solution to this problem. One proposal has been to substitute other divalent oxides for BaO. This remedy has not found favor. It has also been proposed to add either Al.sub.2 O.sub.3 or La.sub.2 O.sub.3 to the glass composition. Current commercial practice employs a small amount of Al.sub.2 O.sub.3 in the glass composition.
While the durability problem has been alleviated, other problems have persisted. In general, these problems arise in delivery and forming of molten glass. The glass has a tendency to devitrify during the delivery process. Also, a high liquidus temperature has necessitated delivering the glass at a relatively high temperature of about 950.degree. C. A lower delivery temperature would facilitate forming by allowing faster mold cooling, higher press speeds, and closer dimensional control of the product.
Accordingly, a need has arisen for a dense barium crown glass having improved physical properties to facilitate delivery and forming of the glass. In particular, a lower liquidus temperature and a greater viscosity at the liquidus would be desirable. At the same time, it is necessary to maintain optical properties, as well as resistance to chemical and moisture attack. It is a basic purpose of the present invention to meet these needs.